As active materials to be used for positive electrodes of a non-aqueous electrolyte secondary battery, lithium-containing transition metal oxides, such as lithium cobalt oxide, have been widely used. These metal oxides are produced by baking a raw material mixture at a high temperature. The raw material mixture comprises a cobalt salt, a lithium salt or the like. However, an active material exhibits basicity when a lithium salt remains therein; in order to prevent this, there has been proposed that a cobalt content be made higher than a lithium content in a raw material mixture (cf. Japanese Patent No. 3252433).
With a cobalt content made higher than a lithium content in a raw material mixture, on the other hand, a by-product, such as cobalt oxide (CO3O4), may be mixed into an active material to cause a decrease in capacity of a battery or gas generation. There has hence been proposed as opposed to the above proposal that a lithium content be made higher than a cobalt content in a raw material mixture (cf. Japanese Laid-Open Patent Publication No. Hei 6-338323).
Although studies are underway on control of a ratio of cobalt and lithium contained in a raw material mixture, as thus described, conventional active materials essentially have insufficient thermal stability. On this account, an active material prepared at any ratio tends to decompose while oxygen generates, when a battery goes into an overcharged state.
It is thought that stabilization of a crystal structure of an active material can be sought by baking a raw material mixture at a high temperature. When an active material is synthesized at an extremely high temperature, however, oxygen is released to cause occurrence of oxygen deficiency in the crystal structure of the active material. When the synthesis is conducted at a high temperature of 1000° C. or higher, for example, oxygen deficiency occurs in a large amount in the active material. A non-aqueous electrolyte secondary battery produced using such an active material undergoes a capacity decrease and significant deterioration in cycle characteristic.
Meanwhile, as another means for improving thermal resistance of lithium cobalt oxide, one of the typical positive electrode active materials, a proposal has-been made that part of the Co atoms in LiCoO2 be replaced with a different metal element (cf. Japanese Laid-Open Patent Publication No. 2002-198051). For example, addition of magnesium to a raw material mixture involves a certain degree of capacity decrease, but an active material crystal structure is stabilized. It is thereby possible to obtain an active material having relatively high thermal stability. Nevertheless, owing to concern about the aforesaid occurrence of oxygen deficiency in an active material crystal structure, synthesis of an active material at a substantially high temperature is not under study.
In the case where the raw material mixture is added with magnesium, a lithium amount becomes relatively small and an active material thus comes to contain MgO besides Co3O4. MgO may cause gas generation within a battery, and furthermore, when MgO is isolated, the stabilization of a crystal structure in line with the capacity decrease cannot be obtained.